This runs into what I'm calling the "who ordered that" problem, because this represents a retreat from a stronger commitment to privacy, and is not a conception of privacy that anyone was asking for, or that satisfies anyone who is concerned about privacy.
I don't want my interest in sci-fi to be made to conflict with my preference from buying locally, and influence campaigns urging me buy books through Preferred LArge Retailer and pushing me toward that clash are a problem whether the data powering them is personal or fed into an abstracted anonymized group.
And depersonalized profiling that "knows" I can be sorted into a specific "type of guy" bucket may involve learning things about me that I don't want to be inputs into marketing. They can still, for instance, make inroads into judgements about things like self esteem (e.g. colognes and beauty products), financial precarity, and can work to socialize groups into consumerist self-conceptions. They probably can be used to make inroads into classic forms of privacy violations like "looking to buy a home" or "trying to get pregnant" or other such aspects of identity that I don't want marketing to touch.
See this submission from earlier this month: Everyone knows your location: tracking myself down through in-app ads (26 days ago, 1957 points) – https://www.hackerneue.com/item?id=42909921
This situation isn't perfect, but I disagree that this is particularly weasely or disingenuous. It's not black & white and there are meaningful differences here.
I think the assumption of 'selling data' and primary concern from most users is the sale of their identifiable personal data - i.e. telling advertisers "this user is interested in X", using their privileged position as a browser to track and collect that information. This is absolutely what Facebook is doing when they sell your data, for example.
The description here is suggesting that Firefox are still committed to never doing that or anything similar. That is the main thing I'd want to know, so that's great.
However, it sounds like they may be selling generic anonymous data in some way - for example telling Pocket what percentage of people use the Pocket extension, or telling Google what percentage of people change their search engine away from Google. Both of those are cases where you can imagine they might receive significant extra income from partners given that data, and they feel this is reasonable but means they can technically no longer say the 'never sell your data'.
You could consider that level of data sharing problematic of course. That said, there is spectrum of problems here, and personally (and I think for most people) I am much more concerned about the tracking & distribution of actual personal identifiable data than I am about generic metrics like those, if that is what's happening (unfortunately, they haven't explained much further so this is still somewhat speculation - I fully agree more precise language would be very helpful).